1. Field of the Invention
The present invention relates to a method and an apparatus for detecting irregularities in an optical fiber, especially during a drawing process for forming the optical fiber.
2. Description of Related Art
It is known to employ a control method in which variations in the external diameter of the optical fiber are fed back to a pulling capstan. FIG. 3 (Prior Art) is a schematic diagram of a conventional drawing apparatus utilizing such a control method. An optical fiber preform 1 is melted in a heating furnace 2, pulled by a capstan 4, and formed into an optical fiber 3. The external diameter of the optical fiber 3 is measured by an external diameter measuring instrument 5 which is located just downstream of the heating furnace 2. Thereafter, the optical fiber 3 is coated with a resin by a coating unit 6, and then pulled through a resin curing furnace 7.
The external diameter measurements from the external diameter measuring instrument 5 are fed back to the control system of the pulling rate of the pulling capstan 4 so that the optical fiber has a consistent, predetermined external diameter (normally 125 .mu.m).
When the external diameter of the optical fiber varies due the presence of contaminants, such as microscopic particulate matter, the variation in external diameter is detected by the external diameter measuring instrument 5. That is because the feedback system described above cannot compensate for such a discrete variation.
However, if bubbles or the like remain in the optical fiber preform over some length as it is drawn, the pulling rate of the pulling capstan 4 is suitably altered to maintain a consistent external diameter of the optical fiber 3. Accordingly, the external diameter measuring instrument 5 cannot detect irregular portions of the optical fiber 3. However, the bubbles may collapse when the optical fiber is drawn, even though the external diameter is controlled. Consequently, the core of the optical fiber deforms and causes increased transmission loss. Therefore, such deformed portions of the optical fiber must be detected in order to be removed.